1. Field of the Invention
The present invention relates to screw compressors. It finds particular application in a single screw compressor having a main rotor and two or more meshing gate rotors.
2. Description of Related Art and Introduction
Screw compressors have become increasingly popular for refrigeration and air conditioning applications in recent years. Their high reliability, small size and weight for a given capacity, make these compressors ideal for use in packaged chiller units. Environmental issues are increasingly important and thus also efficient operation of these chillers.
The single screw compressor is a known type, comprising a single main rotor 100 with two meshing gate rotors 110, 115. An example of these rotors is shown in FIG. 1. The single main rotor 100 has a number of helical screw threads 105, sometimes referred to as “flutes”, which are cut with a globoid (or hour glass) shape to the roots of these threads. The threads 105 have a relatively large cross section at an input end 120 and a significantly smaller cross section at a discharge end 125.
Suction gas enters the flutes 105 at the large openings at the input ends 120, in a generally axial direction with respect to the main rotor 100. The gas is then sealed into the flutes 105 by the gate rotors 110, 115 and casing (not shown) as the rotor assembly 100, 110, 115 rotates, the discharge ends 125 of the flutes 105 normally being closed by the casing. Continued rotation causes the teeth of the gate rotors 110, 115 to progress along the flutes 105 causing a reduction in volume and thus an increase in pressure. The compressor is so designed that when the desired pressure increase has been reached the flute opens to a discharge port in the casing and continued rotation causes the refrigerant gas to be driven out through the discharge port. The design allows for this compression process to be mirrored on both sides of the main rotor 100 by the use of two gate rotors 110, 115.
FIG. 1 shows a compression process in three different rotational positions. In a first position, shown to the left in FIG. 1, a gas-filled flute 105 has a relatively large volume, indicated by a dotted area. As the input end 120 is sealed by a tooth of a gate rotor 115 which begins to move along the gas-filled flute 105 during rotation of the rotor assembly 100, 110, 115, the volume of the gas-filled flute 105 reduces, as shown in the middle of FIG. 1. The volume of the gas-filled flute 105 reaches a minimum just as its discharge end 125 comes level with a discharge port (not shown) in the casing. This last rotational position is shown to the right in FIG. 1. The gas expands as it is released through the discharge port. This process is repeated for each consecutive flute 105.
It is not always necessary or desirable to run a compressor at full capacity. In the past it has been sufficient to produce units that operate efficiently at full load, but it is well known that for most of the time the average chiller is used at between 25% and 75% of full capacity. The importance of high efficiency in these operational bands is recognised by both ARI and Eurovent. The Eurovent index ESEER, a rating very similar to ARI IPLV, provides a realistic overall efficiency figure by applying weighting coefficients to efficiencies at various part loads. The following table shows these weighting coefficients across a set of ESEER parameters:
Part loadAir Water WeightingRatioTemperature (° C.)Temperature (° C.)Coefficients1003530 3% 75302633% 50252241% 25201823%
It can be seen that the weighting coefficients are much higher for the part load ratios 25% to 75%.
Various unloading mechanisms have been developed in order to provide compression at a reduced rate. In the screw compressor, the integral arrangement that has become virtually universal nowadays is some form of axial slide. These are used to adjust two factors: capacity and volume ratio. Capacity is controlled by determining the position along a flute 105 at which gas is taken in. The volume ratio is the relationship between the volume of trapped gas at the start of a compression process in a flute 105 and the volume of the trapped gas when it first starts to discharge from the flute 105. An arrangement utilised in most single screw compressor types incorporates two axially moving slides, sitting in a recess inside the casing, adjacent to and sealing part of the compressor rotor. In the standard arrangement, axial movement of the slides opens or closes ports in the compressor casing to achieve changes in the capacity and the volume ratio. In practice, a bypass port in the casing effectively delays the start of compression and it is this port which is progressively opened or closed to control capacity. A discharge port at the other end of the casing is simultaneously modified to control the volume ratio.
Referring to FIG. 2, a port 205 cut in a slide 200 that's otherwise arranged for capacity control also allows it to control the opening of a discharge port in the casing. Thus the slide 200 performs two distinct functions, the first to adjust the capacity, the second to maintain the appropriate volume ratio. Careful design of the slide 200 can produce arrangements that either maintain a fixed volume ratio over most of the operating range or provide a changing volume ratio to match anticipated changes in the operating pressures at part load.
In a further refinement, it is possible to separate these two functions by dividing the slide into two separate sections.
In the single screw compressor with two gate rotors 110, 115 as shown in FIG. 1, there are two sets of compression processes that occur at the same time, one for each gate rotor 115 as it sweeps through flutes 105 on one side of the main rotor 100. Each set of compression processes is therefore provided with an unloading slide 200. It is known to use such slides asymmetrically, to give different loading in their respective compression processes at the same time. The ability to provide different loading in each of at least two compression processes can produce a compressor whose operation is more efficient when part-loaded.
Preferably, a first of the at least two slides is operable to move between a fully loaded position and a fully unloaded position while a second of the at least two slides is operable to move to any of a range of partially loaded positions. Such an arrangement allows the compressor to operate through a wide loading range, potentially extending from very low loading through to fully loaded.